The long-range purpose of this project is to examine the neural basis of the adaptive mechanisms responsible for maintaining appropriate performance levels in the vestibulo-ocular reflex. Our present concern is to determine where the modifiable elements are located through single unit recordings in the vestibuloocular reflex pathways of (a) normal monkeys and (b) monkeys whose vestibuloocular reflex has been modified as a result of prolonged optical reversal of vision. No significant differences were found between the two groups of animals in the gain, phase or resting discharge rates of their semicircular canal primary afferents, hence, vestibular efferents are not involved in the adaptive process. Signals carried by the Purkinje output cells in the floccular lobes of the vestibular cerebellum were different in the two groups of monkeys but the nature of these changes suggested that they were incidental to modifications occurring elsewhere in the vestibulo-ocular pathways.